1. Field of the Invention
The present invention relates to a device for controlling inductive load, which is employed to control the inductive loads (such as motor, DC electromotor and etc) and can prevent the contact of the relay switch from producing sparks, thus prolonging the service and increasing the reliability of the relay switch.
2. Description of the Prior Art
With the development of inductive load system, all kinds of technologies for controlling inductive load and the related products have become the indispensable part of many precision machines.
A conventional inductive load (such as: motor, DC motor and etc) controlling system is one of the technologies for controlling inductive load and is shown in FIG. 1 (U.S. Pat. No. 6,487,062 B1), wherein the circuit system includes an inductive load 10 (namely a DC motor), two relays 11 and 12, and two relay switches 13 and 13a. The relays 11 and 12 serve to control the two relay switches 13 and 13a and the inductive load 10. The two relay switches 13 and 13a each has a electrically common contact 131 and 131a, a normal close contact 132 and 132a, a normal open contact 133 and 133a, and an elastic metal piece 134 and 134a. The inductive load 10 is connected to the electrically common contacts 131, 131a of the relay switches 13 and 13a, and the normal close contacts 132 and 132a of the two relay switches 13 and 13a are to the electrically common contacts 131 and 131a, respectively, via the elastic metal pieces 134 and 134a (the electrically common contacts 131 and 131a are electrically connected to the normal close contacts 132 and 132a at normal condition). The circuit is further connected with a metal oxide semiconductor field effect transistor (MOSFET) and a DC supply 15. The relay switches 13 and 13a are parallel to each other. The MOSFET 14 and the DC supply 15 are connected in series to the relay switches 13 and 13a in such a manner that a positive electrode of the DC supply 15 is connected to the relay switches 13 and 13a and the negative electrode of the DC supply 15 is connected to the MOSFET 14.
When the relay 11 produces a field coil current and the elastic metal piece 134 of the relay switch 13 is moved from the normal close contact 132 to the normal open contact 133, namely, the electrically common contact 131 is electrically connected to the normal open contact 133. At this moment, the current generated from the positive electrode of the DC supply 15 will be transmitted to the negative electrode of the DC supply 15 after passing through the normal open contact 133 of the relay 11, the elastic metal piece 134, the electrically common contact 131, the inductive load 10, the electrically common contact 131a of the relay switch 13a, the elastic metal piece 134a, the normal close contact 132a and the MOSFET 14, respectively. Meanwhile, the GATE of the MOSFET 14 must provide a turn-on voltage, so that the inductive load 10 can run forwardly.
On the other hand, when the relay 11 produces a field coil current and the elastic metal piece 134a of the relay switch 13a is moved from the normal close contact 132a to the normal open contact 133a, namely, the electrically common contact 131a is electrically connected to the normal open contact 133a. At this moment, the current generated from the positive electrode of the DC supply 15 will be transmitted to the negative electrode of the DC supply 15 after passing through the normal open contact 133a of the relay switch 13a of the relay 12, the electrically common contact 131 of the relay switch 13 of the relay 11, the elastic metal piece 134, the normal close contact 132 and the MOSFET 14, respectively. Meanwhile, the GATE of the MOSFET 14 must provide a turn-on voltage, so as to make the inductive load 10 run in reverse.
This conventional inductive load controlling method still has some shortcomings as follows:
First, the relays 11 and 12 must be provided with a predetermined magnetized voltage in order to make the GATE of the MOSFET 14 produce a control voltage, therefore, the control circuit will be too complicated, and accordingly the production cost will be high.
Second, the GATE of the MOSFET 14 must produce a control voltage, however this control voltage will be produced firstly, and then if the voltage of the relay 11 or 12 is produced after the control voltage, it will cause damage to the contact 133 or 133a of the relay 11 or 12 because of the instantly increased current.
Third, as shown in FIG. 1, the user must prepare another control system in order to ensure that the relays 11 and 12 are turned on before the MOSFET 14, and this also needs a time-delay device.
Fourth, when the relays 11 and 12 are turned on synchronously, the current previously flowed in the load will produce sparks at the relay switches 13, 13a of the relays 11 and 12, and will cause damage.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.